In the case of power transistors, particularly Darlington circuits, one often needs to be able to locate an interruption in the connections and lines in the load circuit so that appropriate counter-measures or auxiliary measures can be introduced. It is generally known to use a measuring resistor, which is traversed by the load current flow or by a portion of the same, for this purpose. To keep the power loss dissipation generated across the measuring resistor as low as possible, the measuring resistor, as a rule, has a very small resistance value, so that in a proper state, the rated current produces only a slight drop in voltage and a small power loss. Also, for those cases in which a very small portion of the primary current flows through the measuring resistor, often only a slight voltage drop is nevertheless required to ensure that the conditions in the main circuit and in the measuring circuit do not differ too greatly.
A device for recognizing such a line interruption must also function reliably in response to current values that lie far below the nominal value, often even by several orders of magnitude below this value. In this manner, the voltages to be evaluated become extremely small and the necessary measurement techniques become very complicated and expensive.